Content-based image compression

ABSTRACT

A technique for selecting portions of a multi-resolution medical image data set to be stored and the portions of the multi-resolution medical image data set to be discarded in order to reduce the overall amount of image data that is stored for each image data set. The selection is based on the clinical purpose for obtaining the medical image data. The clinical purpose for obtaining the medical image is used to select a segmentation algorithm to segment the image data into several regions based on their relevance to the purpose for obtaining the image data. A mask is created from the segmented data. The mask is used to assign data compression quality factors to corresponding regions in the image data. The image data in each region is compressed based on their assigned quality factor. The image data that is highly relevant is compressed with lossless data compression. The image data in regions of lesser relevance are compressed with lossy data compression. The image data that is of no relevance for the purpose that the image data was obtained is discarded.

BACKGROUND

The invention relates generally to the field of medical image datastorage. More particularly, the invention relates to a technique forreducing the amount of medical image data of a medical image data setthat is stored in long-term storage.

Picture archiving and communications systems, or PACS, have become anextremely important component in the management of digitized image data,particularly in the field of medical imaging. Such systems oftenfunction as central repositories of image data, receiving the data fromvarious sources, such as medical imaging systems. The image data isstored and made available to radiologists, diagnosing and referringphysicians, and other specialists via network links. Improvements inPACS have led to dramatic advances in the volumes of image dataavailable, and have facilitated loading and transferring of voluminousdata files both within institutions and between the central storagelocation and remote clients.

In the medical diagnostics field, depending upon the imaging modality,digitized data may be acquired and processed for a substantial number ofimages in a single examination, each image representing a large data setdefining discrete picture elements (pixels) of a reconstructed image, orvolume elements (voxels) in three dimensional data sets. Computedtomography (CT) imaging systems, for example, can produce numerousseparate images along an anatomy of interest in a very short examinationtimeframe. Other imaging modalities are similarly capable of producinglarge volumes of useful image data, including magnetic resonance imaging(MRI) systems, digital X-ray systems, X-ray tomosynthesis systems,ultrasound systems, positron emission tomography (PET) systems, and soforth. Ideally, all such images are stored centrally on the PACS, andmade available to the radiologist for review and diagnosis.

Various techniques have been proposed and are currently in use foranalyzing and compressing large data files, such as medical image datafiles. Image data files typically include streams of data descriptive ofimage characteristics, typically of intensities or other characteristicsof individual pixels or voxels in the reconstructed image. In themedical diagnostic field, these image files are typically created duringan image acquisition, encoding or processing (e.g., reconstruction)sequence, such as in an X-ray, MRI, CT, or other system, or in aprocessing station designed to process image data from such systems. Theimage data may be subsequently processed or reprocessed, such as toadjust dynamic ranges, or to enhance certain features shown in theimage, for storage, transmittal and display.

While image files may be stored in raw and processed formats, many imagefiles are quite large, and would occupy considerable disc or storagespace. Moreover, an almost exponential increase in the resolutions ofimaging systems has occurred and which appears will continue into thefuture, leading to the creation of ever larger image files, typicallyincluding more data as a result of the useful dynamic range of theimaging system, the size of the matrix of image pixels and voxels, andthe number of images acquired per examination. In addition, theprocessing and memory requirements for current PACS systems for newclinical applications and techniques is beginning to tax current systemcapabilities, such as the ever increasing clinical needs for volumetricdata sampled over time and for the use of multiple energy volumes forbetter visualization of anatomical and functional features.

In addition to occupying large segments of available memory, large imagefiles can be difficult or time consuming to transmit from one locationto another. In a typical medical imaging application, for example, ascanner or other imaging device will typically create raw data which maybe at least partially processed at the scanner. The data is thentransmitted to other image processing circuitry, typically including aprogrammed computer, where the image data is further processed andenhanced. Ultimately, the image data is stored either locally at thesystem, or in the PACS for later retrieval and analysis. In all of thesedata transmission steps, the large image data file must be accessed andtransmitted from one device to another.

Current image handling techniques include compression of image datawithin the PACS environment to reduce the storage requirements andtransmission times. Such compression techniques generally, however,compress entire files, including descriptive header information whichcould be useful in accessing or correlating images for review. Moreover,current techniques may not offer sufficiently rapid compression anddecompression of image files to satisfy increasing demands on systemthroughput rates and access times. Finally, alternative compression anddecompression techniques do not offer the desired compression ratios, incombination with rapid compression and decompression in a client-serverenvironment.

Another drawback of existing compression techniques is the storage,access and transmission of large data files even when a user cannot ordoes not desire to view the reconstructed image in all available detail.For example, in medical imaging, extremely detailed images may beacquired and stored, while a radiologist or physician who desires toview the images may not have a view port capable of displaying theimages in the resolution in which they are stored. Thus, transmission ofthe entire images to a remote viewing station, in relatively timeconsuming operations, may not provide any real benefit and may slowreading or other use of the images. Furthermore, only certain portionsof a medical image may be relevant for diagnosis or treatment. Thus,considerable storage space in a PACS may be allocated to the storage ofmedical image data that is irrelevant for the patient's diagnosis andtreatment. This problem becomes even more acute as imaging systemsachieve greater and greater resolutions, which correspond to a need foreven more data storage space.

There is a need, therefore, for an improved image data compression anddecompression technique which provides rapid compression anddecompression of image files, and which obtains improved compressionratios and transmission times. In addition, there also is a need for atechnique which permits compressed image data files to be created andtransmitted in various resolutions or sizes, depending upon thebandwidth and desired or available resolution on a client side.Furthermore, there is a particular need for a technique to enable imagedata storage systems to accommodate the increase in data required tostore medical images obtained with ever increasing resolutions ofimaging systems.

BRIEF DESCRIPTION

A technique for selecting portions of a multi-resolution medical imagedata set to be stored and the portions of the multi-resolution medicalimage data set to be discarded in order to reduce the overall amount ofimage data that is stored for each image data set. The selection isbased on the clinical purpose for obtaining the medical image data. Theclinical purpose for obtaining the medical image is used to select asegmentation algorithm to segment the content of the image data intoseveral regions based on their relevance to the purpose for obtainingthe image data. A mask is created from the segmented data. The mask isused to assign data compression quality factors to corresponding imageblocks in the image data. The image data in each region is compressedbased on their assigned quality factor. The image data in regions ofmost relevance to the purpose for obtaining the image data arecompressed with lossless data compression. The image data in regions oflesser relevance to the purpose for obtaining the image data arecompressed with lossy data compression. The image data in regions of norelevance to the purpose for obtaining the image data are discarded.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic view of an exemplary imaging system, in this casea computed tomography (“CT”) imaging system, designed to implement theenhanced image data storage scheme in accordance with an exemplaryembodiment of the present technique;

FIG. 2 is a diagrammatical representation of a picture archiving andcommunication system, or PACS, for receiving and storing image data fromthe imaging system of FIG. 1, in accordance with an exemplary embodimentof the present technique;

FIG. 3 is a block diagram of an exemplary technique for selectivelystoring medical image data based on the content of the image, inaccordance with an exemplary embodiment of the present invention;

FIG. 4 is a diagrammatical representation of the technique of FIG. 3, inaccordance with an exemplary embodiment of the present invention;

FIG. 5 is an exemplary medical image taken along the saggittal plane ofthe knee, in accordance with an exemplary embodiment of the presentinvention;

FIG. 6 is an exemplary segmented image data from FIG. 5, in accordancewith an exemplary embodiment of the present invention; and

FIG. 7 is an exemplary image mask created from segmented image of FIG.5, in accordance with an exemplary embodiment of the present invention;

DETAILED DESCRIPTION

Referring now to FIG. 1, the present invention will be described as itmight be applied in conjunction with an exemplary imaging system, inthis case a computed tomography (CT) imaging system. In general,however, it should be borne in mind that the present techniques may beused with image data produced by any suitable imaging modality. In atypical application, the imaging system may be designed both to acquireoriginal image data and to process the image data for display andanalysis is presented,. As noted below, however, in certain applicationsthe image data acquisition and subsequent processing (e.g, for thetransformations and compression described below) may be carried out inphysically separate systems or work stations. The illustrated embodimentof the CT imaging system 20 has a frame 22, a gantry 24, and an aperture(imaging volume or CT bore volume) 26. A patient table 28 is positionedin the aperture 26 of the frame 22 and the gantry 24. The patient table28 is adapted so that a patient 30 may recline comfortably during theexamination process.

The illustrated embodiment of the CT imaging system 20 has an X-raysource 32 positioned adjacent to a collimator 34 that defines the sizeand shape of the X-ray beam 36 that emerges from the X-ray source 32. Intypical operation, the X-ray source 32 projects a stream of radiation(an X-ray beam) 36 towards a detector array 38 mounted on the oppositeside of the gantry 24. All or part of the X-ray beam 36 passes through asubject, such as a patient 30, prior to impacting the detector array 38.It should be noted that all or part of the x-ray beam 36 may traverse aparticular region of the patient 30, such as the liver, pancreas, heart,and so on, to allow a scan of the region to be acquired. The detectorarray 38 may be a single slice detector or a multi-slice detector and isgenerally formed by a plurality of detector elements. Each detectorelement produces an electrical signal that represents the intensity ofthe incident X-ray beam 36 at the detector element when the x-ray beam36 strikes the detector array 38. These signals are acquired andprocessed to reconstruct an image of the features within the patient 30.

The gantry 24 may be rotated around the patient 30 so that a pluralityof radiographic views may be collected along an imaging trajectorydescribed by the motion of the X-ray source 32 relative to the patient30. In particular, as the X-ray source 32 and the detector array 38rotate along with the gantry 24, the detector array 38 collects photonsresulting from X-ray beam attenuation at the various view anglesrelative to the patient 30 and produces signals or data representativeof the incident photons. Data collected from the detector array 38 thenundergoes pre-processing and filtering to condition the data torepresent the line integrals of the attenuation coefficients of thescanned patient 30. The processed data, commonly called projections, arethen filtered and back projected to formulate an image of the scannedarea. Thus, an image or slice is acquired which may incorporate, incertain modes, less or more than 360 degrees of projection data, toformulate an image.

Rotation of the gantry 24 and operation of the X-ray source 32 arecontrolled by a system controller 40, which furnishes both power andcontrol signals for CT examination sequences. Moreover, the detectorarray 38 is coupled to the system controller 40, which commandsacquisition of the signals generated in the detector array 38. Thesystem controller 40 may also execute various signal processing andfiltration functions, such as for initial adjustment of dynamic ranges,interleaving of digital image data, and so forth. In general, systemcontroller 40 commands operation of the imaging system 20 to executeexamination protocols and to process acquired data. In the presentcontext, system controller 40 also includes signal processing circuitry,typically based upon a general purpose or application-specific digitalcomputer, associated memory circuitry for storing programs and routinesexecuted by the computer, as well as configuration parameters and imagedata, interface circuits, and so forth. The system controller 40includes a gantry motor controller 42 that controls the rotational speedand position of the gantry 24 and a table motor controller 44 thatcontrols the linear displacement of the patient table 28 within theaperture 26. In this manner, the gantry motor controller 42 rotates thegantry 24, thereby rotating the x-ray source 32, collimator 34 and thedetector array 38 one or multiple turns around the patient 30.Similarly, the table motor controller 44 displaces the patient table 28,and thus the patient 30, linearly within the aperture 26. Additionally,the X-ray source 32 may be controlled by an X-ray controller 46 disposedwithin the system controller 40. Particularly, the X-ray controller 46may be configured to provide power and timing signals to the X-raysource 32.

In the illustrated embodiment, the system controller 40 also includes adata acquisition system 48. In this exemplary embodiment, the detectorarray 38 is coupled to the system controller 40, and more particularlyto the data acquisition system 48. The data acquisition system 48typically receives sampled analog signals from the detector array 38 andconverts the data to digital signals for subsequent processing. An imagereconstructor 50 coupled to the computer 52 may receive sampled anddigitized data from the data acquisition system 48 and performshigh-speed image reconstruction. Alternatively, reconstruction of theimage may be done by the computer 52. Once reconstructed, the imageproduced by the imaging system 10 reveals internal features of thepatient 30.

The data collected by the data acquisition system 48, or thereconstructed images, may be transmitted to the computer 52 and to amemory 54. It should be understood that any type of memory to store alarge amount of data may be utilized by such an exemplary imaging system10. Also the computer 52 may be configured to receive commands andscanning parameters from an operator via an operator workstation 56typically equipped with a keyboard and other input devices. An operatormay control the CT imaging system 20 via the operator workstation 56.Thus, the operator may observe the reconstructed image and other datarelevant to the system from computer 52, initiate imaging, and so forth.

The CT imaging system 20 also has a display 58 that is coupled to theoperator workstation 56 and the computer 52 and may be utilized by auser to observe the reconstructed image, as well as to provide aninterface for control of the operation of the CT imaging system 20. Inthis embodiment, a printer 60 is present to enable a hard copy of amedical image to be printed. In the illustrated embodiment, the CTimaging system 20 is coupled to a picture archiving and communicationssystem (PACS) 62 via the operator workstation 56 for long-term storageof image data. It should be noted that the PACS 62 may be coupled to aremote system 64, such as radiology department information system (RIS),hospital information system (HIS) or to an internal or external network,so that others at different locations may gain access to the image andto the image data. However, access to the image data may also beobtained remotely through the PACS 62.

It should be further noted that the computer 52 and operator workstation56 may be coupled to other output devices, such as a standard or specialpurpose computer monitor and associated processing circuitry. One ormore operator workstations 56 may be further linked in the CT imagingsystem 20 for outputting system parameters, requesting examinations,viewing images, and so forth. In general, displays, printers,workstations, and similar devices supplied within the CT imaging system20 may be local to the data acquisition components, or may be remotefrom these components, such as elsewhere within an institution orhospital, or in an entirely different location, linked to the imagingsystem CT via one or more configurable networks, such as the Internet,virtual private networks, and so forth.

As noted above, it should be borne in mind that the CT system referredto herein is merely one exemplary source of image data that may behandled in accordance with the present techniques. Most such systemswill include operator interfaces and software specifically adapted toacquire image data and to at least partially process the data inaccordance with the specific physics of the imaging modality. Indeed,other arrangements of CT systems, other reconstruction techniques, andso forth may give rise to image data that may be managed as describedherein.

Referring generally to FIG. 2, an exemplary embodiment of a PACS 62 forreceiving, compressing and decompressing image data is presented. In theillustrated embodiment, the CT imaging system 20 is used for short-termstorage of image data only. Memory 54 of the CT imaging system 20 islimited and cannot be used to store image data with any degree ofpermanence, particularly when the system is used to carry outexaminations for a large number of patients in a clinic, hospital orother institution. For example, data space occupied by old image datamay be written over by new image data. The PACS 62 is used for long-termstorage of medical image data. In the illustrated embodiment, PACS 62receives image data from CT imaging system 20, as well as several otherseparate imaging systems designated by reference numeral 66. As will beappreciated by those skilled in the art, the imaging systems may be ofvarious type and modality, such as MRI systems, PET systems, radiofluoroscopy (RF), computed radiography (CR), ultrasound systems, digitalX-ray systems, X-ray tomosynthesis systems, ultrasound systems, and soforth. Moreover, the systems may include processing stations ordigitizing stations, such as equipment designed to provide digitizedimage data based upon existing film or hard copy images. It should alsobe noted that the systems supplying the image data to the PACS may belocated locally with respect to the PACS, such as in the sameinstitution or facility, or may be entirely remote from the PACS, suchas in an outlying clinic or affiliated institution. In the latter case,the image data may be transmitted via any suitable network link,including open networks, proprietary networks, virtual private networks,and so forth.

PACS 62 includes one or more file servers 68 designed to receive andprocess image data, and to make the image data available fordecompression and review. File server 68 receives the image data throughan input/output interface 70. Image data may be compressed in routinesaccessed through a compression/decompression interface 72. As describedmore fully below, compression/decompression interface 72 serves tocompress the incoming image data rapidly and optimally, whilemaintaining descriptive image data available for reference by fileserver 68 and other components of the PACS. Where desired,compression/decompression interface 72 may also serve to decompressimage data accessed through the file server 68. The file server 68 isalso coupled to internal clients, as indicated at reference numeral 74,each client typically including a work station at which a radiologist,physician, or clinician may access image data from the server,decompress the image data, and view or output the image data as desired.Clients 74 may also input information, such as dictation of aradiologist following review of examination sequences. Similarly, fileserver 68 may be coupled to one or more interfaces, such as a printerinterface 76 designed to access and decompress image data, and to outputhard copy images via a printer 78 or other peripheral.

A database server 80 is used to associate image data, and other workflow information within the PACS, by reference to one or more fileservers 68. In the presently contemplated embodiment, database server 80may include cross-referenced information regarding specific imagesequences, referring or diagnosing physician information, patientinformation, background information, work list cross-references, and soforth. The information within database server 80 serves to facilitatestorage and association of the image data files with one another, and toallow requesting clients to rapidly and accurately access image datafiles stored within the system. Similarly, file server 68 is coupled toone or more archives 82, such as an optical storage system, which serveas repositories of large volumes of image data for backup and archivingpurposes. Techniques for transferring image data between file server 68,and any memory associated with file server 68 forming a short-termstorage system, and archive 82, may follow any suitable data managementscheme, such as to archive image data following review and dictation bya radiologist, or after a sufficient time has lapsed since the receiptor review of the image files.

In the illustrated embodiment, other components of the PACS system orinstitution may be integrated with the foregoing components to furtherenhance the system functionality. For example, acompression/decompression library 84 is coupled tocompression/decompression interface 72 and serves to store compressionroutines, algorithms, look up tables, and so forth, for access byinput/output interface 70 (or other system components) upon execution ofcompression and decompression routines (i.e. to store various routines,software versions, code tables, and so forth). In practice,compression/decompression interface 72 may be part ofcompression/decompression library 84. Library 84 may also be coupled toother components of the system, such as internal clients 74 or printerinterface 76, serving similarly as a library or store for thecompression and decompression routines and algorithms. Althoughillustrated as a separate component, it should be understood thatcompression/decompression library 84 may be included in any suitableserver or memory device, including within file server 68. Moreover, codedefining the compression and decompression processes described below maybe loaded directly into compression/decompression interface 72 and/orcompression/decompression library 84, or may be loaded or updated vianetwork links, including wide area networks, open networks, and soforth.

Additional systems may be linked to the PACS, such as directly to server80, or through interfaces such as input/output interface 70. In theembodiment illustrated in FIG. 2, a radiology department informationsystem or RIS 86 is linked to file server 68 to facilitate exchanges ofdata, typically cross-referencing data within database server 80, and acentral or departmental information system or database. Similarly, ahospital information system or HIS 88 may be coupled to database server80 to similarly exchange database information, workflow information, andso forth. Where desired, such systems may be interfaced through dataexchange software, or may be partially or fully integrated with the PACSsystem to provide access to data between the PACS database and radiologydepartment or hospital databases, or to provide a singlecross-referencing database. Similarly, external clients, as designatedat reference numeral 90, may be interfaced with the PACS to enableimages to be viewed at remote locations. Such external clients mayemploy decompression software, or may receive image files alreadydecompressed by compression/decompression interface 72. Again, links tosuch external clients may be made through any suitable connection, suchas wide area networks, virtual private networks, and so forth.

In the illustrated embodiment, the PACS 62 provides for multi-resolution(or multi-size) image data compression. Where a user does not desire toview a full image with maximum resolution, or where the user view portis limited, such multiresolution image compression facilitates transferof a reduced size image to the user for viewing, with excellent imagequality. Moreover, the multiresolution image compression may allow auser to view a reduced size or reduced resolution image relativelyrapidly, and to “zoom” on the image thereafter by transfer of only aportion of the compressed data corresponding to components of thegreater sized image not already transferred. The additional data is thenprocessed and combined with the reduced size image data to obtain thelarger sized image. In addition, the technique described below utilizespurpose-driven image data storage to reduce the amount of stored imagedata associated with an image stored in the PACS 62.

It should be noted that the processing and storage of the image data asdescribed below may be performed in the PACS 62, or in any othersuitable system component or components. The processing will typicallybe embodied in computer code that can be stored and executed on any oneor more than one of the computers of the acquisition the PACS, anoperator workstation, server, and so forth, so long as the system iscapable of performing the computations involved.

The multi-resolution implementation may be based partially upon losslessinteger wavelet decomposition. Specifically, as will be recognized bythose skilled in the art, wavelet decomposition involves a dyadicfiltering and sub-sampling process. This creates a hierarchical set ofsub-bands. As will be discussed in more detail below, a wavelettransformed image data set includes low frequency components along withhigh frequency components, which may be considered as noise orvariations from the low frequency components. A single level waveletdecomposition results in a decomposed data set which includes one lowfrequency sub-band LL, along with three high frequency ones LH, HL, andHH. Subsequent decomposition may be considered to produce a further dataset in which the low frequency sub-band is further decomposed into a setof sub-bands, including a low frequency band, along with threeadditional high frequency sub-bands. The wavelet transformationtechnique may be carried out on two dimensional or three dimensional (orhigher dimension) data sets.

Referring generally to FIG. 3, a technique is presented for selectivelystoring image data within an image based on the content of the image,represented generally by reference numeral 92. The illustrated techniqueenables the amount of medical image data that is stored in the PACS 62for each medical image taken to be reduced. By knowing the purpose forobtaining the medical image data, it is possible to identify the contentof the medical image that is highly relevant and the content of themedical image that is of lesser relevance, or even irrelevant. In theillustrated embodiment, the content of the medical image that is highlyrelevant is compressed with lossless data compression, which maintainsits full resolution. The content of the medical image that is lessrelevant is compressed with lossy data compression, which results in alower resolution for those regions of the image. The content of themedical image that is not relevant is discarded completely. After datacompression, the contents of the image that are either highly relevantor of a lesser relevance are stored in long-term storage in the PACS 62.Because not all of the image data for an image is stored at fullresolution, the amount of data storage space required by the image isreduced.

As will be discussed in more detail below, the different contents of themedical image are assigned a quality factor that establishes therelevance, and, therefore, the type of data compression at which thatportion of the image data will be compressed, or whether it will becompressed at all in the case of irrelevant image data. In thisembodiment, any portion of medical image data that is assigned a qualityfactor corresponding to highly relevant image data is compressed withlossless compression. Any portion of medical image data that is assigneda quality factor that corresponds to less relevant image data iscompressed with lossy compression and stored with less than fullresolution. Finally, any portion of medical image data that is assigneda quality factor that corresponds to irrelevant image data is discardedand not compressed, at all.

The compression quality factor that a particular region or anatomicalfeature is assigned is based on the clinical purpose for obtaining themedical image data. Depending on the clinical purpose of the scan, ananatomical feature may be of great interest in one medical image and oflittle or no interest in another medical image. For example, if thepurpose of the scan is to enable a radiologist to look for tumors in thebrain, anatomical features other than the brain would be of lesserinterest. Therefore, in this example, brain tissue image data is highlyrelevant and would have a quality factor assigned to it to reflect thatit is highly relevant. On the other hand, blood vessel image data is ofa lesser relevance and would be assigned a quality factor to reflect itslesser relevance. Similarly, cerebrospinal fluid is irrelevant and wouldhave a quality factor assigned to it to reflect its irrelevance.

The technique calls for identifying the clinical purpose for obtainingthe medical image data, as represented generally by block 94. There maybe any of a myriad of clinical purposes for obtaining the medical imagedata, such as an angiogram, a mammogram, a perfusion, to locate a tumor,an aneurysm, blocked blood vessels, etc. In addition, the scan may beperformed for any portion of the body. A system operator may select thepurpose from a menu or list or purposes, thereby directing the system toautomatically execute the steps of the technique.

To obtain medical image data the patient is scanned using the CT imagingsystem 20, represented generally by block 96. In the illustratedembodiment, the image data that is obtained is three-dimensional imagedata. However, additional dimensions of image data corresponding tovarious other types of data, such as time, temperature, pressure, etc.,may be obtained, as well. In the illustrated embodiment, the medicalimage data that is obtained by scanning the patient is obtained at asingle resolution, which is the highest resolution available from the CTimaging system 20. As noted above, this technique is applicable for usewith imaging systems other than the CT imaging system 20. For imagingtechnologies based upon other physical parameters, the intensity, color,brightness or another parameter or characteristic of each pixel or voxelwill be based the unique physics of that technology (e.g., net magneticmoment for MRI systems, gamma radiation received for PET imaginessystems, and so forth).

The full resolution medical image data is then accessed for furtherprocessing before long-term storage, represented generally by block 98.The medical image data may be processed by the CT imaging system 20and/or the PACS 62. However, in this embodiment, the CT imaging system20 is not used for long-term storage of medical image data. Instead,long-term storage of medical image data occurs in the PACS 62. However,in other embodiments of the present technique, an imaging system may beused for long-term storage of medical image data, rather than a PACS.

A segmentation algorithm is selected to segment the contents of themedical image data based on the clinical purpose of the scan, asrepresented by block 100. For example, if the purpose of the scan is toenable a radiologist to examine the brain for tumors, a segmentationalgorithm could be selected that is adapted to segment the varioustissues of the head, such as brain tissue, bone, skin, etc. As anotherexample, if the purpose of the scan was to examine the condition of thecartilages of the knee, a segmentation algorithm could be selected thatis adapted to segment cartilage from other tissues, such as bone,muscle, and ligaments. A copy of the original image data may be used forsegmentation purposes. Once selected, the segmentation algorithm is thenused to segment the image data, represented generally by block 102. Inthe illustrated embodiment, the segmented image data isthree-dimensional image data.

In the illustrated embodiment, the segmented image data is used tocreate a mask having at least three groups based on their relevance tothe clinical purpose for obtaining the image data, represented generallyby block 104. In this embodiment, the segmentation algorithm is operableto classify the segmented portions of the image data into three groupsbased on their relevance to the clinical purpose for obtaining themedical image data. The three groups are image data having highlyrelevant content, image data having less relevant content, and imagedata having no relevant content. For example, a segmentation algorithmmay segment brain tissue, bone, and ventricles filled with cerebrospinalfluid within an image of the brain. In this embodiment, in addition tosegmenting the portions of the image data, the segmentation programwould also establish the degrees of relevance for the brain tissue,bone, and ventricles filled with cerebrospinal fluid based on theclinical purpose for obtaining the medical image data. If the purposefor obtaining the image data is for identifying brain tumors, the braintissue would be identified as highly relevant and the bone would beidentified as less relevant, or perhaps irrelevant. Similarly, theventricles filled with cerebrospinal fluid would be identified asirrelevant. If there were another purpose for obtaining the image data,the degrees of relevance for the various tissues might change. The maskcomprises three-dimensional image data in this embodiment. In addition,instead of the segmentation algorithm, a separate algorithm may be usedto create the mask in another embodiment.

The mask is comprised of image blocks that are assigned quality factorsthat correspond to the three groups of image data. A different qualityfactor is assigned to each group of image data to distinguish them. Forexample, an image block in the mask that corresponds to highly relevantimage data may be assigned with a quality factor of “two.” Similarly,each portion of the mask that corresponds to less relevant image datamay be assigned with a quality factor of “one” and an image block in themask that corresponds to irrelevant image data may be assigned a qualityfactor of “zero.” However, these numbers are arbitrary and may varydepending upon the system in which the technique is utilized. The maskis stored with a resolution appropriate for the image block size of thecompression, as represented generally by block 106.

A quality factor is then assigned to each image block in the image databased on the quality factors assigned to the mask, represented generallyby block 108. The quality factors of the image blocks of the mask aretransferred to the corresponding image blocks in the image data. In theexample described above, all image blocks of the highly relevant imagedata are assigned a quality factor of “two”, the image blocks of theless relevant image blocks are assigned a quality factor of “one”, andthe image blocks of the irrelevant image data are assigned a qualityfactor of “zero”.

The medical image data is then compressed using the assigned qualityfactors, represented generally by block 110. Using the quality factors,the image blocks corresponding to the highly relevant image data arecompressed losslessly. Image blocks corresponding to the less relevantimage data are compressed with lossy compression. Finally, image blockscorresponding to the irrelevant image data are discarded and notcompressed. In the illustrated embodiment, the PACS 62 compresses theimage data. The PACS identifies the quality factor for each image blockprior to data compression and then uses the appropriate algorithm ortool to compress the block of image data. The image data may also bedecomposed into a plurality of resolution levels using wavelettransformation.

After data compression, the compressed image and the compressed mask arestored in long-term storage, represented generally by block 112. Fromthere, the image data may be retrieved from long-term storage forviewing at any time. Because not all of the image data was stored withlossless compression, the amount of data storage space needed to storeeach image is reduced. However, because all of the highly relevant imagedata is saved with lossless data compression, there is no loss of imagedata or resolution in the highly relevant regions of the image. Inaddition, the less relevant image data is provided for clarity, but at alower resolution that the highly relevant image data. The resolution ofthe less relevant image data may be set to a desired value.

Referring generally to FIG. 4, a diagrammatical representation of thetechnique for selectively storing image data based on the content of theimage is presented, referenced generally by reference numeral 114. Anoriginal image 116 is used to create a segmented image 118. As alsonoted above, a segmentation algorithm is used to segment the originalimage based on the clinical purpose for the exam. The segmented image118 is then used to create a mask 120. The mask 120 is then used toassign different quality factors to various regions of the originalimage 122, as represented by block 122. The various portions of theimage data are then processed based on their assigned quality factors,represented by block 124. Those portions of the image data having highlyrelevant or less relevant values are compressed according to theirassigned quality factor, represented by block 126. As noted above, thoseportions of the image data having highly relevant status are compressedwith lossless resolution, while those portions having less relevantstatus are compressed with lossy compression at less than fullresolution. The compressed data is then stored in long-term storage, asrepresented by block 126. Those portions of the image data havingquality factors corresponding to irrelevant image data are discarded andnot stored in long-term storage, represented by block 130. Thus, imagesthat are stored in long-term storage will not have any image data forthe discarded regions. As a result, images produced from the image datastored in long-term storage are composed of portions, or regions, havingdifferent resolutions. The most relevant portions of the image have fullresolution, while images with less relevance have lower resolutions. Inaddition, the image may be decomposed using wavelet decomposition toproduce different levels of image resolution. Each resolution level insuch a multi-resolution image set would have a full resolution portion(for that resolution level) and a lower resolution portion (for thatresolution level).

Referring generally to FIG. 5, a reconstructed image slice of apatient's knee is presented, referenced generally by reference numeral132. In this image, the clinical purpose for obtaining the image data isto examine the condition of the femoral cartilage 134 within the knee132. In addition to the femoral cartilage 134, various bones 136 of theknee 134, including portions of the femur and tibia, are shown. Inaddition, other soft tissues 138 are shown, such as muscles andligaments.

Referring generally to FIG. 6, a segmented image of the femoralcartilage 134 is presented. In addition, other tissues of the knee aresegmented and used to form the mask.

Referring generally to FIG. 7, a mask, represented generally byreference numeral 140, of the knee of FIG. 5 is presented. In thisembodiment, the mask 140 has three regions representing the three groupsof tissues, each region having its own quality factors. The image blocks142 corresponding to the femoral cartilage are assigned a quality factorto signify that these image blocks have the most relevance. In thisparticular embodiment, the image blocks 142 corresponding to the femoralcartilage are the most relevant and have an assigned quality factor of“two”. The image blocks 144 corresponding to the bones of the knee areirrelevant and are assigned a quality factor of “zero” to signify thatthese image blocks are irrelevant. Finally, the image blocks 146corresponding to the soft tissues of the knee are assigned a qualityfactor of “one” to signify that these image blocks are relevant, butless relevant than the femoral cartilage.

As discussed above, the mask 140 is used to assign quality factors tothe knee 132 of FIG. 5 to direct the compression of the image data. Inthis embodiment, those portions of the image of the knee 132 that areassigned the quality factor having a value of “two” are compressed withlossless compression. In this example, that corresponds to the femoralcartilage and some of its adjacent tissue. Other portions of the knee132 that are assigned quality factors having a value of “one” arecompressed with lossy compression. In this example, that corresponds tothe soft tissues 138 of the knee 138. As a result, the soft tissues havea lower resolution than the femoral cartilage 134 when viewed after thedata has been compressed. Finally, the portions of the knee 132 that areassigned a quality factor of “zero” are simply discarded and not storedin long-term storage. In this example, that corresponds to bone. As aresult, less data storage space is required to store the image than ifall of the regions of the image had been stored at full resolution.However intended are presented in full resolution. In this example, thatis the femoral cartilage and some adjacent tissue.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A computer-implemented method for processing image data, comprising:accessing image data obtained by an imaging system; segmenting the imagedata based on a purpose for obtaining the image data; creating a masklabeled with image data compression information based on segmented imagedata; compressing the image data based on the image data compressioninformation; and storing the compressed image data in long-term storage.2. The computer-implemented method for processing image data as recitedin claim 1, wherein a first portion of the image data is compressed withlossless compression based on the image data compression information. 3.The computer-implemented method for processing image data as recited inclaim 2, wherein a second portion of the image data is compressed withlossy compression based on the image data compression information. 4.The computer-implemented method for processing image data as recited inclaim 3, wherein a third portion of the image data is discarded based onthe image data compression information.
 5. The computer-implementedmethod for processing image data as recited in claim 2, wherein a secondportion of the image data is discarded based on the image datacompression information.
 6. The computer-implemented method forprocessing image data as recited in claim 1, comprising: transferringthe image data compression information from the mask to the image data.7. The computer-implemented method for processing image data as recitedin claim 1, wherein the image data compression information comprises aplurality of compression quality factors, each compression qualityfactor corresponding to a region in the mask and its relevance to otherregions in the mask.
 8. The computer-implemented method for processingimage data as recited in claim 1, wherein segmenting the image databased on a purpose for obtaining the image data comprises selecting asegmentation algorithm from a plurality of segmentation algorithms basedon the purpose for obtaining the image data.
 9. The computer-implementedmethod for processing image data as recited in claim 1, wherein theimage data comprises three-dimensional image data.
 10. Thecomputer-implemented method for processing image data as recited inclaim 1, comprising creating the mask based on a purpose for obtainingimage data.
 11. The computer-implemented method for processing imagedata as recited in claim 10, wherein creating the mask comprisesselecting the purpose for obtaining the image data from among aselection of purposes.
 12. The computer-implemented method forprocessing image data as recited in claim 10, comprising storing themask in long-term data storage.
 13. A computer-implemented method forstoring image data, comprising: accessing image data obtained by animaging system; segmenting the image data based on a purpose forobtaining the image data; creating a mask labeled with image datacompression information based on segmented image data; compressing afirst portion of the image data with lossless compression and a secondportion of the image data with lossy compression based on the image datacompression information; and storing the compressed image data inlong-term storage.
 14. The computer-implemented method for storing imagedata as recited in claim 13, comprising: transferring the image datacompression information from the mask to the image data.
 15. Thecomputer-implemented method for storing image data as recited in claim13, comprising: discarding a third portion of the image data based onthe image data compression information.
 16. The computer-implementedmethod for storing image data as recited in claim 13, wherein segmentingthe image data based on a purpose for obtaining the image data comprisesselecting a segmentation algorithm from a plurality of segmentationalgorithms based on the purpose for obtaining the image data.
 17. Thecomputer-implemented method for storing image data as recited in claim16, wherein the segmentation algorithm segments the image data into aregion of most relevance, a region of lesser relevance, and a region ofno relevance based on the purpose for obtaining the image data.
 18. Thecomputer-implemented method for storing image data as recited in claim17, wherein the image data compression information comprises a pluralityof compression quality factors, a first compression quality factorcorresponding to the region of most relevance, a second compressionquality factor corresponding to the region of lesser relevance, and athird compression quality factor corresponding to the region of norelevance.
 19. A computer-implemented method for storing image data,comprising: accessing image data obtained by an imaging system;segmenting the image data based on a purpose for obtaining the imagedata; creating a mask labeled with image data compression informationbased on segmented image data; compressing a first portion of the imagedata based on the image data compression information; discarding asecond portion of the image data based on the image data compressioninformation; and storing the compressed image data in long-term storage.20. The computer-implemented method for storing image data as recited inclaim 19, wherein compressing a first portion of the image datacomprises compressing the first portion of the image data with losslesscompression based on the image data compression information.
 21. Thecomputer-implemented method for storing image data as recited in claim20, comprising: compressing a third portion of the image data based onthe image data compression information.
 22. The computer-implementedmethod for storing image data as recited in claim 19, wherein segmentingthe image data based on a purpose for obtaining the image data comprisesselecting a segmentation algorithm from a plurality of segmentationalgorithms based on the purpose for obtaining the image data.
 23. Thecomputer-implemented method for storing image data as recited in claim22, wherein the segmentation algorithm segments the image data into aregion of most relevance, a region of lesser relevance, and a region ofno relevance based on the purpose for obtaining the image data.
 24. Thecomputer-implemented method for storing image data as recited in claim23, wherein the image data compression information comprises a pluralityof compression quality factors, a first compression quality factorcorresponding to the region of most relevance, a second compressionquality factor corresponding to the region of lesser relevance, and athird compression quality factor corresponding to the region of norelevance.
 25. A system for processing image data, comprising: means foraccessing image data obtained by an imaging system; means for segmentingthe image data based on a purpose for obtaining the image data; meansfor creating a mask labeled with image data compression informationbased on segmented image data; means for compressing the image databased on the image data compression information; and means for storingthe compressed image data in long-term storage.
 26. A system for storingimage data, comprising: means for accessing image data from an imagingsystem; means for segmenting the image data based on a purpose forobtaining the image data; means for creating a mask labeled with imagedata compression information based on segmented image data; means forcompressing a first portion of the image data with lossless compressionand a second portion of the image data with lossy compression based onthe image data compression information; and means for storing thecompressed image data in long-term storage.
 27. A system for storingimage data, comprising: means for accessing image data obtained by animaging system; means for segmenting the image data based on a purposefor obtaining the image data; means for creating a mask labeled withimage data compression information based on segmented image data; meansfor compressing a first portion of the image data based on the imagedata compression information; means for discarding a second portion ofthe image data based on the image data compression information; andmeans for storing the compressed image data in long-term storage.
 28. Amachine-readable medium for processing medical image data, comprising:code operable to access image data obtained by an imaging system; codeoperable to segment the image data based on a purpose for obtaining theimage data; code operable to create a mask labeled with image datacompression information based on segmented image data; code operable tocompress the image data based on the image data compression information;and code operable to store the compressed image data in long-termstorage.
 29. A machine-readable medium for processing medical imagedata, comprising: code operable to access image data obtained by animaging system; code operable to segment the image data based on apurpose for obtaining the image data; code operable to create a masklabeled with image data compression information based on segmented imagedata; code operable to compress a first portion of the image data withlossless compression and a second portion of the image data with lossycompression based on the image data compression information; and codeoperable to store the compressed image data in long-term storage.
 30. Amachine-readable medium for processing medical image data, comprising:code operable to access image data obtained by an imaging system; codeoperable to segment the image data based on a purpose for obtaining theimage data; code operable to create a mask labeled with image datacompression information based on segmented image data; code operable tocompress a first portion of the image data based on the image datacompression information; code operable to discard a second portion ofthe image data based on the image data compression information; and codeoperable to store the compressed image data in long-term storage.